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The weldability of plain and inoculated 6061 aluminum processed with gas metal arc directed energy deposition (GMA-DED) was evaluated and compared to wrought 6061. Autogenous gas tungsten arc welds of varying heat inputs were made, and the degree of solidification cracking was evaluated. The degree of cracking in the inoculated 6061 material was lower than that of plain GMA-DED and wrought 6061. Microstructure characterization revealed that the welds on the inoculated 6061 produced fine, equiaxed grains, whereas the plain 6061 showed coarse, columnar grains. A combination of heat transfer and solidification models were employed to predict the solidification morphology of the 6061 welds, which closely matched the experimental results in all cases. A model was developed to understand the effect of grain morphology on solidification cracking, and it was found that equiaxed grains shifted the critical liquid film range for cracking to lower solid fractions where thermal stresses are the lowest. However, cracking can be caused if sufficient external stresses are applied when the critical liquid film thickness is present during solidification of the equiaxed grain structure. This work provides insight into the role grain size and morphology control can have in suppressing solidification cracking of other aluminum alloys. 

Abstract The solidification microstructures of plain and inoculated 6061 aluminum builds manufactured with gas metal arc-directed energy deposition were studied with a combination of models and experiments. Electron back-scatter diffraction (EBSD) showed that the plain 6061 build had large, columnar grains with intergranular solidification cracking, while the inoculated build had a near-equiaxed, fine grain microstructure with no solidification cracks. By combining EBSD and energy dispersive spectrometry, the inoculated build has been shown to have exhibited globular growth while the non-inoculated build displayed a dendritic microstructure. A combination of heat transfer and modified grain morphology models were employed to predict the solidification morphology of the 6061 builds, which closely matched experimental results. A modification is proposed to the criterion marking the transition from globular to dendritic growth that better matches experimental results in this work. The results of this study are expected to provide improved methods to predict solidification microstructure for the development of new materials and processing parameters for additive manufacturing.